It is known that the object of roasting an agro-food product is either to destroy certain harmful features of the product or to cause formation of certain flavor features, with drying of the product.
Traditional roasting processes consist either of directly attacking either the grains or a metallic enclosure which contains the grains with a flame, or else passing a current of preheated gas into this enclosure. In both cases, the grains are mechanically stirred to improve the uniformity of treatment or to favor the contact between phases. At any rate, in the direct flame attack process, the treatment of the grains is not homogenous. With coffee for example, the calcinated aromatic oils appear at the surface of the grains which then cannot be preserved: the structure of the grain is made very fragile and numerous breaks are produced during the treatment. It is also established that the desquamation or peeling of some grains occurs when the grains enter too rapidly into contact with the heated wall of the enclosure. In the other aforementioned process, the heat yield is very mediocre because the exchange between the gas and the grains is not good, and this type of treatment leads to great consumption of energy.
In a recent attempt to improve the heat exchanges between the gas and the grains in this process, the grains have been put in suspension or "fluidized" in the current of hot gas: this technique assures excellent irrigation of the fluidized bed by the hot gas and obtains an intense stirring of the grains without necessitating mechanical agitation. Large dimension grains, for example coffee grains of which the dimensions are on the order of 5 to 8 millimeters to be fluidized, require that the gas circulate at high speeds, greater than the minimum fluidization velocity which is on the order of 1.2 m/s for green coffee grains. This causes several inconveniences. In the first place, the flows of larger passage necessitate provision of accessory installations for gas circulation, which are costly and cumbersome. Also, the cross section of the enclosure in which the torrefaction or roasting is to be effected must remain relatively small so as not to require excessive gas flows, which is not compatible with practical operation. Consequently, the time unit yields of products treated by fluidization are low. And the correct control of the roasting time imposes a discontinuous progress of the treatment, charge by charge. The discontinuous function makes the process for difficult to automate and increases the personnel necessary; it also leads to heat losses during the starting phases.
Whatever the quality of the contact between the gaseous phase and the solid phase, the coefficient of thermal transfer between a gas and a solid is always moderate, which limits the possibilities of exchanges between the two phases, and does not permit satisfactory use of the heat potential of the gas.
The present invention proposes a remedy of the aforementioned inconveniences with a new roasting process which will permit uniform roasting of the grains in remarkably economical conditions, particularly concerning energy usage.
One object of the invention is to permit continuous treatment.
Another object is to reduce the cost and cumbersomeness of the necessary installations.